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United States Patent |
5,342,404
|
Alt
,   et al.
|
August 30, 1994
|
Implantable medical interventional device
Abstract
An implantable medical interventional device responds to detection of
cardiac activity of the patient indicative of ventricular tachycardia (VT)
or fibrillation (VF) by successively applying to the patient's heart
selected ones of different electrical waveforms for treatment to break the
VT or VF, respectively, until the monitored cardiac activity indicates
that the treatment has been successful. An accelerometer adapted to sense
position and movement of the patient along three mutually orthogonal axes
acts as an activity status sensor to detect physical activity and
inactivity of the patient to complement the detection of the patient's
cardiac activity for confirming that a detected VT is a pathologic
tachycardia rather than a physiologic tachycardia, and in response to such
confirmation the device selects an appropriate electrical waveform for the
treatment. The device is capable of recognizing from detected physical
activity the occurrence of a sudden vigorous movement associated with a
syncope by calculating variances relative to the mean of the detected
physical activity, and of responding to such recognition by selecting a
defibrillating waveform for immediate application to the patient's heart.
A scoring system based on assigned weightings of subparameters of detected
cardiac activity and detected physical activity or inactivity is used to
determine the likelihood that a physiologic tachycardia, a pathologic
tachycardia, or fibrillation is being detected.
Inventors:
|
Alt; Eckhard (Ottobrunn, DE);
Matula; Marcus (Munich, DE);
Mestre; Edgar (Munich, DE)
|
Assignee:
|
Intermedics, Inc. (Angleton, TX)
|
Appl. No.:
|
863092 |
Filed:
|
April 3, 1992 |
Current U.S. Class: |
607/6; 600/595; 607/14; 607/19 |
Intern'l Class: |
A61N 001/39 |
Field of Search: |
128/782
607/6,14,18,19
|
References Cited
U.S. Patent Documents
4771780 | Sep., 1988 | Sholder | 128/782.
|
4836218 | Jun., 1989 | Gay et al. | 128/782.
|
5014698 | May., 1991 | Cohen | 607/6.
|
5031614 | Jul., 1991 | Alt | 607/19.
|
5040535 | Aug., 1991 | Mann et al. | 607/19.
|
5097831 | Mar., 1992 | Lekhulm | 607/18.
|
5233984 | Aug., 1993 | Thompson | 128/782.
|
Foreign Patent Documents |
0360412 | Mar., 1990 | EP | 607/18.
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: O'Connor Cavanagh
Claims
We claim:
1. An implantable medical interventional device for treatment of pathologic
tachycardias, comprising:
first sensor means adapted to detect cardiac activity of the patient in
which the interventional device is implanted for producing a first
electrical signal representative of such cardiac activity including heart
rate of the patient,
second sensor means adapted to detect movement and lack of movement
indicative of physical activity and inactivity of the patient for
producing a second electrical signal representative of the patient's
activity status,
generator means for selectively generating electrical waveforms to be
applied to the patient's heart to break a pathologic tachycardia, and
control means setting a predetermined heart rate and responsive to said
first and second electrical signals as complementary criteria for
distinguishing a heart rate of the patient exceeding the predetermined
rate during a period of physical activity by the patient indicative of a
pathologic tachycardia requiring treatment from a physiologic tachycardia
attributable to patient activity, and including activation means
responsive to a decision by said control means that the patient is
experiencing a pathologic tachycardia for triggering said generator means
to generate an electrical waveform having characteristics selected to
break the tachycardia.
2. The implantable medical interventional device of claim 1, wherein:
said first sensor means comprises a sensor for detecting and generating a
signal representative of the patient's ECG.
3. The implantable medical interventional device of claim 1, wherein:
said second sensor means comprises an accelerometer.
4. The implantable medical interventional device of claim 3, wherein:
the accelerometer comprises means to detect orientation and movement of the
patient in three dimensions.
5. The implantable medical interventional device of claim 4, wherein said
means to detect orientation and movement of the patient in three
dimensions comprises:
first and second mercury ball sensors, each including a mercury ball and a
multiplicity of spaced-apart electrodes in a plane over which the
respective mercury ball may roll in response to movement of the patient
for intermittently establishing electrical connections with various ones
of the electrodes, and
said electrode planes of said first and second mercury ball sensors being
orthogonally oriented relative to one another to enable detection of
movement and orientation of the accelerometer along three different
orthogonal axes.
6. The implantable medical interventional device of claim 5, wherein said
control means further includes:
detection means responsive to electrical closures and openings of the
spaced-apart electrodes in each of said first and second mercury ball
sensors as the respective mercury ball makes and breaks electrical
connections with various ones of said spaced-apart electrodes during
rolling of the ball, and
distinction means for calculating the quotient of the standard deviation
divided by the mean of the closures and openings over a predetermined
interval of time to detect sudden brief movement by the patient consistent
with a syncope for enhancing the decision by said control means that the
patient is experiencing a pathologic tachycardia.
7. The implantable medical interventional device of claim 3, further
including:
a case housing said device, and
means mechanically isolating said accelerometer from the case to render the
accelerometer substantially unresponsive to pressure on the case.
8. The implantable medical interventional device of claim 1, wherein:
said second sensor means comprises integrated electronic circuit means
responsive to orientation and movement thereof for producing said second
electrical signal.
9. The implantable medical interventional device of claim 8, wherein:
said integrated electronic circuit means is a silicon based
mechanoelectrical converter.
10. The implantable medical interventional device of claim 9, wherein:
said mechanoelectrical converter is selected from the group consisting of
piezoresistive, piezoelectric and piezocapacitive converters.
11. The implantable medical interventional device of claim 1, wherein:
said control means is further responsive to said complementary criteria for
deciding whether the patient is experiencing ventricular fibrillation,
said generator means includes storage capacitor means for storing a level
of electrical charge suitable for defibrillation of the patient's heart,
and
said activation means is responsive to a decision by said control means
that the patient is experiencing ventricular fibrillation for causing said
storage capacitor means to be charged to said level for delivery within
said electrical waveform when the generator means is triggered by the
activation means.
12. The implantable medical interventional device of claim 1, wherein said
control means further includes:
calculating means responsive to said second electrical signal
representative of the patient's activity status for calculating the
standard deviation relative to the mean of said second electrical signal
over a predetermined time interval for enhancing discrimination of
pathologic tachycardia from physiologic tachycardia by said control means.
13. In combination with an implantable defibrillator adapted to selectively
deliver electrical waveform therapies to a patient's heart in response to
detection of cardiac activity indicative of ventricular tachycardia or
fibrillation, the improvement comprising:
control means for discriminating between pathologic tachycardia and
physiologic tachycardia during the patient's engagement in physical
activity, said control means including
implantable complementary sensor means for detecting movement indicative of
physical activity of the patient,
computing means responsive to detected physical activity and confirming
detected cardiac activity of the patient indicative of pathologic
tachycardia for recognizing a sudden brief movement by the patient as
syncope, and
generator means responsive to recognition of syncope by said computing
means for generating a predetermined antitachycardia electrical waveform
therapy to be delivered to the patient's heart to revive the patient.
14. The improvement of claim 13, wherein:
said complementary sensor means includes means for producing an electrical
signal in response to detecting movement indicative of physical activity
of the patient, and
said computing means includes means for calculating the quotient of the
standard deviation divided by the mean of variations of said electrical
signal with the detected movement over time.
15. The improvement of claim 13, wherein:
said complementary sensor means comprises accelerometer means for sensing
movement of the patient along three mutually orthogonal axes.
16. The improvement of claim 15, wherein the accelerometer means comprises:
a pair of orthogonally oriented mercury ball sensors each including
spaced-apart planar electrodes and a mercury ball for contacting various
ones of said spaced-apart electrodes to intermittently establish and
disrupt electrical connections therewith over time and to establish
particular electrical connections therewith at any given time upon
positioning of the respective mercury ball in response to movements and
static position of the patient.
17. A method of timing the delivery of antitachycardia and defibrillating
electrical therapies to a patient's heart from an automatic implanted
defibrillator device, comprising the device-implemented steps of:
detecting the current status of the patient's physical activity while
simultaneously detecting the patient's cardiac activity to recognize a
ventricular tachycardia while the patient is engaged in physical activity,
determining the degree of probability that the recognized ventricular
tachycardia is a pathologic tachycardia rather than a physiologic
tachycardia by weighting the detected cardiac activity and the detected
status of physical activity of the patient as separate indicia of the
degree of probability that a pathologic tachycardia is occurring, and
delivering an antitachycardia electrical therapy to the patient's heart
when the weighting indicates a predetermined degree of probability that a
pathologic tachycardia is occurring.
18. The method of claim 17, further including the device-implemented steps
of:
detecting whether the antitachycardia electrical therapy delivered to the
patient's heart was effective to terminate the tachycardia, and, if not,
whether the tachycardia is accelerating into ventricular fibrillation, and
delivering defibrillating electrical therapy to the patient's heart if the
previously delivered antitachycardia electrical therapy was ineffective to
terminate the tachycardia coupled with detection of acceleration into
ventricular fibrillation.
19. The method of claim 17, further including the device-implemented steps
of:
monitoring the cardiac activity and the current status of physical activity
or inactivity of the patient following the delivery of an antitachycardia
electrical therapy to determine whether that delivered therapy succeeded
in terminating the pathologic tachycardia, and
delivering another antitachycardia electrical therapy different from the
last delivered therapy when the monitoring indicates that the last
delivered therapy did not terminate the pathologic tachycardia.
20. The method of claim 19, wherein the step of delivering another
antitachycardia electrical therapy includes:
selecting said another antitachycardia electrical therapy to be delivered
from the group consisting of a single burst of stimulating pulses, plural
bursts of stimulating pulses, a train of rapid stimulation pulses, a
single low energy shock, plural low energy shocks, and a relatively higher
energy shock, and
generating the selected another antitachycardia electrical therapy for
delivery to the patient's heart.
21. An implantable medical device for intervention in response to the
implant patient experiencing a syncope, comprising:
first sensor means for detecting electrical cardiac activity of the patient
and for producing a first electrical signal representative thereof,
second sensor means for detecting posture, position and physical activity
of the patient and for producing a second electrical signal representative
thereof,
discrimination means responsive to said first and second electrical signals
for discriminating between a physiologic tachycardia and a pathologic
tachycardia of the patient's heart,
control means responsive to discrimination of a tachycardia by said
discrimination means as a pathologic tachycardia for prescribing one among
a plurality of different electrical therapy waveforms for treatment of the
pathologic tachycardia, and
generator means responsive to the prescribing of one of said electrical
therapy waveforms by said control means for generating the prescribed
waveform for application to the patient's heart,
said discrimination means including computing means responsive to said
second electrical signal representative of detected physical activity of
the patient by said second sensor means for recognizing therefrom the
occurrence of a collapse of the patient associated with a syncope,
said control means including means responsive to said recognizing by said
computing means for prescribing defibrillating waveform from among said
plurality of electrical therapy waveforms for treatment of the syncope.
22. The implantable medical device of claim 21, wherein:
said computing means includes means responsive to said second electrical
signal representative of detected physical activity of the patient by the
second sensor means, and representative of detected changes of position of
the patient having a mean value over a time interval and undergoing
variances from the mean value, for calculating from said second electrical
signal the variances relative to the mean value thereof to enhance
recognition of a syncope event.
23. The implantable medical device of claim 22, wherein:
said second sensor means comprises accelerometer means for sensing movement
of the patient along three mutually orthogonal axes.
24. The implantable medical device of claim 21, wherein:
said first electrical signal has a plurality of parameters associated with
at least the patient's heart rate and ECG,
said second electrical signal has a plurality of parameters associated with
at least the patient's posture, position and change thereof, and activity,
and
said computing means includes tallying means responsive to said parameters
of said first and second electrical signals for assigning predetermined
weighted Values to said parameters according to the relative importance of
each Of said parameters in assessing probability that a cardiac event
including at least one of pathologic tachycardia and syncope is occurring
based on detection by the respective sensor means, and for adding the
weighted values of the parameters for determining from the total of said
weighted values over a predetermined interval of time the likelihood that
a syncope is occurring in that interval of time.
25. A method of determining when to apply any of a plurality of available
antitachycardia therapies, including possible defibrillation therapy, to a
patient's heart from an implantable antitachycardia device, comprising the
steps of:
providing said plurality of antitachycardia therapies,
detecting at least two different physiologic parameters of the patient from
which hemodynamic events for treatment with said therapies are
discernible, one of said parameters representing electrical cardiac
activity of the patient,
processing the detected different physiologic parameters to obtain a
variety of subparameters identifying characteristics of the respective
detected different physiologic parameters from which said subparameters
were obtained,
scoring each of the processed subparameters by assigning to it a relative
value according to its importance in signifying the probability of
occurrence of a predetermined hemodynamic event, and
adding the relative values of the processed subparameters to obtain a
cumulative score, and selecting one of the provided plurality of
anti-tachycardia therapies prescribed as best suited for treatment of the
predetermined hemodynamic event when the cumulative score of the processed
subparameters reaches a preestablished level of probability that said
predetermined hemodynamic event is occurring.
26. The method of claim 25, including:
designating movement indicative of physical activity of the patient as
another of the at least two different physiologic parameters to be
detected.
27. The method of claim 26, wherein:
the step of processing the detected different physiologic parameters to
obtain a variety of subparameters is performed to obtain subparameters
including at least one of patient rest, patient exercise, patient
position, and change of patient position for the parameter indicative of
patient movement, and to obtain subparameters including at least one of
absolute heart rate, absence of QRS complexes, relative heart rate change,
heart rate stability, and ECG morphology for the parameter representative
of cardiac activity.
28. A method of controlling the delivery of an appropriate therapy of an
antitachycardia device configured and adapted to be implanted in a
patient, which includes:
monitoring the posture, position and physical activity of the patient by
use of at least two two-dimensional accelerometers carried by the patient
to detect movement of the patient along three mutually orthogonal axes,
detecting the patient's ECG,
processing the detected movement of the patient to obtain information on
posture, position and physical activity of the patient, and processing the
detected ECG of the patient to obtain information thereon,
using the information obtained from both detected movement and detected ECG
and a comparison thereof to distinguish a physiologic tachycardia from a
pathologic tachycardia of the patient's heart, and, if a pathologic
tachycardia is found, triggering the antitachycardia device to deliver a
predetermined therapy therefrom predetermined as most appropriate to
terminate the pathologic tachycardia.
29. A control device for controlling the therapeutic function of an
antitachycardia device configured and adapted to be implanted in a
patient, said control device including:
accelerometer means for detecting state of physical activity of the patient
and at least one of posture and position of the patient, and
ECG means for detecting the ECG of the patient,
verification means for comparing the detected state of physical activity of
the patient and said at least one of posture and position of the patient
against the detected ECG of the patient to distinguish a physiologic
tachycardia from a pathologic tachycardia of the patient's heart, and
trigger means for actuating said antitachycardia device to apply a
preselected antitachycardia therapy to the patient upon determination that
the patient is experiencing a pathologic tachycardia.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable medical devices, and
more particularly to an implantable interventional device which is adapted
to deliver electric impulse or shock therapies to the patient's heart upon
detection of a ventricular tachycardia (VT) or ventricular fibrillation
(VF), and to improvements in apparatus and methods for detecting and
distinguishing pathologic tachycardias from physiologic tachycardias and
for establishing the timing of the delivery of the appropriate therapy
upon detection of pathologic VT or VF.
Sinus heart rates in normal healthy adults typically range upward to 160
beats per minute (bpm) during physical activity or exercise, emotional
stress or excitement, or under the influence of various drugs including
alcohol, caffeine or nicotine. Even higher rates, to 200 bpm may be
experienced during strenuous exercise. Rates exceeding 100 bpm in these
and similar circumstances are physiologic tachycardias. The heart rate of
an individual with a normal healthy cardiovascular system will gradually,
perhaps even quickly, decrease toward his or her customary resting rate
when the factors leading to the increased rate have ceased.
In contrast, pathologic tachycardias are abnormal, arising from
cardiovascular disease or disorders, and require medical treatment and
appropriate therapy. A pathologic tachycardia occurring in the atrial
chamber is usually hemodynamically tolerated because the excitable A-V
junction tissue (between the atrium and ventricle) has a longer refractory
period and slower conductivity than myocardial tissue. Hence, the rapid
atrial contractions during atrial tachycardia typically will not induce
correspondingly rapid ventricular contractions, but rather a ventricular
rate of one-half or even one-third in the A-V conduction. Cardiac output
remains relatively strong with a ventricular rhythm within or close to
sinus rate.
However, pathologic tachycardia in the ventricles, the main pumping
chambers of the heart, is not well tolerated because of the diminished
cardiac output attributable to only partial filling of the chambers with
oxygenated blood between the rapid contractions. Moreover, ventricular
tachycardia (VT) tends to accelerate spontaneously to ventricular
fibrillation (VF), in which the myocardial contractions become random and
uncoordinated. Unlike atrial fibrillation, which is generally not
life-threatening because of the relatively small percentage of cardiac
output contributed by the atria, VF is characterized by the loss of
synchronous contractions of the tissue in the main pumping chambers. The
resulting drop in cardiac output will lead to death in minutes unless
adequate cardiac output is restored within that interval.
Atrial tachycardia is relatively common, but patients who are symptomatic
or at high risk may be treated with drugs, antitachycardia pacemakers, or
in some extreme cases, including patients who suffer from atrial
fibrillation, by performing a surgical A-V block and a ventricular
pacemaker implant. The antitachycardia pacemakers, also used in patients
who suffer VT, generally operate on the principle of overstimulating the
heart (at a programmed rapid rate or variable rates) to suppress the
ectopic activity that leads to premature atrial or ventricular
contractions. Only pulses of relatively low energy content may be required
to provide the desired stimulation. Interestingly, previous techniques
used for terminating atrial flutter include high rate pacing of the atrium
exceeding the flutter rate in an attempt to trigger atrial fibrillation,
and spontaneous rapid reversion to normal sinus rhythm. In a technique
sometimes referred to as cardioversion, tachycardia is broken by
delivering higher energy electrical shocks to the heart. Unfortunately,
antitachycardia and cardioversion therapies used for terminating VT can
cause acceleration into VF.
Defibrillators are employed to apply one or more high energy electrical
shocks to the heart to overwhelm the uncoordinated contractions of the
various sections of the myocardial tissue and reestablish organized
spreading of action potentials from cell to cell, and thereby restore
synchronized contractions of the ventricles. Automatic implantable
defibrillators were described in the literature at least as early as 1970,
in separate articles of M. Mirowski et al. and J. Schuder et al. Steady
innovations proposed since that time have included automatic implantable
defibrillators which perform multiple functions of antitachycardia,
cardioversion and defibrillation, and where appropriate, demand
bradycardia pacing. In general, the desire is to use one or more pulse
sequence or low level shock therapies for breaking VT before it
spontaneously accelerates into VF, and, if that fails or if VF occurs
without preliminary pathologic tachycardia, to resort to a high energy
defibrillating shock.
The shocks, both lower energy for antitachycardia and high energy for
defibrillation, are typically delivered from one or more output storage
capacitors in the implanted device which are of sufficient size to store
the electrical charge necessary for these functions. Energy requirements
generally range from as little as 0.05 joule to up to 10 joules for
cardioversion, and from 5 joules to about 40 joules for defibrillation,
depending on the patient, the nature of the electrical waveform applied,
and the efficiency of the energy transfer through the electrodes and into
the heart tissue. It is imperative, particularly where VF is occurring,
that the required energy be available at the time the shock is to be
delivered. Multiphasic shocks have been found effective, and in any event,
it is customary to provide a preset delay between successive shocks, and
to inhibit further shocks when return to normal rhythm is detected.
As used in this specification, the terminology "shock" or "shocks" may
include any pulse-type waveform, whether single phase or multiphase, which
is delivered as antitachycardia, cardioverting or defibrillating therapy
to a patient's heart in an effort to break, interrupt or terminate
pathologic tachycardia or fibrillation and return the pumping action of
the heart to a rate in the normal range; and "interventional device"
includes any antitachycardia pacemaker, cardioverter, defibrillator or
other device or combination thereof which is adapted to be implanted or
otherwise worn by a human or animal subject for the purpose of intervening
to deliver shocks to the heart in response to detection of an abnormally
rapid heart rate. The waveform is not limited to any particular energy
content or range of energy content, and indeed, the therapy may include
burst stimulation or other conventional techniques for applying
stimulation pulses (such as for rapid pacing) to break a VT.
The operation of implantable antitachycardia pacemakers, cardioverters,
defibrillators and similar medical devices raises problems concerning the
timing of delivery of the therapy, such as the timing of charging and
firing of shock-producing output capacitors. In the first instance, it is
necessary to distinguish between physiologic tachycardia and pathologic
tachycardia to assure that the capacitors are not needlessly charged
either from states of full or partial depletion (discharge), and in the
second instance, to guard against premature firing and discharge into the
heart. U.S. Pat. No. 4,114,628 discloses an implantable device which
automatically applies a defibrillating impulse to the patient's heart only
when a predetermined time interval passes without cardiac activity. More
elaborate detection schemes have been suggested. For example, in U.S. Pat.
No. 3,805,795, the defibrillating shock is delivered upon detection of an
absence of both electrical and mechanical physiological functions for a
predetermined time interval.
In general, the prior art devices detect ECG changes and/or absence of a
mechanical function such as rhythmic contractions, pulsatile arterial
pressure, or respiration, and, in response, deliver the appropriate fixed
or programmable therapy. Various types of additional sensors have been
used for the latter functions, including pressure sensors in the heart,
impedance measurements in the heart, flow probes in the aorta, flow probes
in the pulmonary tract, and other extra-pacemaker or extra-defibrillator
sensors, These sensors have not proved to be fully accurate or reliable.
It is a principal object of the present invention to provide improved
techniques for treating ventricular tachycardia and/or fibrillation,
including improved techniques for detecting tachycardia and distinguishing
the normal physiologic type from the abnormal pathologic type.
A related object is the use, for such techniques, of a sensor which may be
housed in the implanted interventional device itself or separately
implanted, to detect position, change of position, and physical activity
(or lack thereof) of the patient, and which applies an algorithm to
reinforce or confirm (or contest or rebut) the ECG criteria, to improve
the reliability of the decision regarding the propriety and timing of the
intervention therapy available from the device.
SUMMARY OF THE INVENTION
According to a principal aspect of the present invention, the decisions on
timing of the application of a selected antitachycardia and/or
defibrillation therapy such as timing of charging and firing of output
capacitors of an implanted interventional device are based, in part, on
sensing resting position, change in position, or activity (physical
movement or exercise) of the patient and generating an electrical signal
representative thereof (referred to throughout this specification as the
"activity status signal" whether the signal denotes movement ranging from
slow to vigorous, or rest in the sense of inactivity or positional changes
ranging from slight to pronounced) which, in conjunction with a
complementary electrical signal from another source, such as from
detection of the patient's cardiac activity (e.g., electrocardiogram or
ECG, hereinafter referred to as the "ECG waveform" or "ECG signal"), is
indicative of VT or VF. Importantly, the two sensed criteria are selected
to reinforce (or possibly rebut) one another, and further proof of VT or
VF (or lack of same) is obtained by calculating the mean and the standard
deviation of the activity status signal. This provides the beneficial
result of considerably better discrimination between a physiologic
tachycardia, attributable for example to strenuous activity or exercise,
and a pathologic tachycardia arising from a cardiac or cardiovascular
disorder.
Using these criteria, the interventional device is adapted to make a
decision to deliver an appropriate electrical shock or shocks (e.g., a
pulse sequence of appropriate energy content) to the patient's heart (the
ventricular myocardium) through implanted epicardial and/or endocardial
electrodes (which may include so-called "patch" electrodes) connected to
the implanted device, or not to do so, according to whether the patient is
undergoing a non-physiologic (pathologic) tachycardia or a physiologic
tachycardia, respectively. The rule by which this decision is made (the
"decision rule") is implemented by appropriate programming of the
interventional device to recognize the significance of confirming or
rebutting evidence from the two signals. Either one of the sensor which
produces the activity status signal (hereinafter referred to as the
"activity status sensor") or the ECG sensor may be regarded as the
primary, secondary or complementary sensor of rapid heart rate criteria
which together evoke the decision rule on whether to apply interventional
therapy.
Preferably, the activity status sensor is located within the case that
houses the implanted interventional device but alternatively, it may be
implanted separately in its own housing. In either event such a sensor is
to be mechanically isolated from the case to avoid false readings
attributable to pressure on the case. The ECG sensor, which receives
signals from the heart via the sensing electrode and associated lead,
would generally be disposed within the interventional device case. The
activity status sensor output is used to complement the output of the ECG
sensor. For example, the ECG signal may indicate a VT of 150 bpm which is
in the range of both pathologic and physiologic tachycardia, but the
activity status sensor detects physical activity, thereby forestalling
delivery of antitachycardia treatment. On the other hand, the ECG may
demonstrate VT or VF at a time when the activity status sensor is
indicating that the patient is not moving, or has just undergone sudden
brief vigorous movement characteristic of a syncope (loss of
consciousness), leading to the decision to trigger prompt therapy. The
decision, therefore, is a reasoned one and is made automatically, and in
the case of origin of a tachycardia, discriminates between physiologic and
non-physiologic.
If, despite the application of therapy from the implanted device, the
tachycardia rate increases, the patient may then lose consciousness and,
if he is standing, collapse. The activity status sensor would detect this
and produce a commensurate output which, upon virtually instantaneous
analysis, would lead to a conclusion that immediate, more stringent
therapy is required. In this particular example, a defibrillating shock
would be triggered.
The decision rule for applying therapy and the nature of the therapy may be
implemented, in part, by calculating the quotient of the standard
variation and the mean of the continuously monitored activity status
sensor signal. This sensor is preferably an accelerometer which may be of
the mercury ball type, or other activity indicator utilizing
mechano-electrical converting element(s) capable of detecting position and
movements of the patient, such as a silicon based piezoelectric,
piezocapacitive, or piezoresistive sensor responsive to gravity and
acceleration. In the preferred embodiment, the activity status sensor
generates signals as a result of patient orientation or movements along
any of three mutually orthogonal axes by means of an orthogonally oriented
pair of mercury ball sensors. Such sensors undergo change in internal
electrical connections as the orientation or acceleration of the sensor
changes. When implanted, the sensor displays a fixed electrical connection
or set of connections for a resting (or at least inactive) patient, or a
varying number and location of electrical connections which are
intermittently disrupted by movements of the patient.
Each mercury ball sensor has an array of electrical contacts across which
the mercury ball may roll, so that the ball makes and breaks adjacent
electrical contacts as it rolls in response to patient movements.
According to an aspect of the invention, the number of variances (standard
deviation) of the making and breaking of electrical connections from the
mean number of electrical connections made and broken within a
predetermined time interval is used to ascertain the nature of the
activity or lack thereof, and whether or not it confirms the detection of
a dramatic hemodynamic event warranting delivery of a more drastic
therapy. Thus, the activity status sensor has the capability to
distinguish patient activity from inactivity, as well as postural changes,
to an extent that the conditions under which a defibrillating shock should
be delivered to the patient's heart are readily discerned.
Alternatively, however, the implantable antitachycardia device may itself
consist at least partially of hybrid electronic circuitry, so that the
accelerometer is advantageously a mechanoelectrical converting element
integrated into the hybrid electronic circuitry for control of the
therapeutic functions of device, and, as noted above, the
mechanoelectrical converting element is silicon based.
Accordingly, another object of the invention is to determine from the
standard deviation relative to the mean of a signal indicative of activity
status of the patient over time, the precise instant at which a
defibrillating shock should be applied to the heart.
The quotient of the standard deviation and the mean or physiologic
statistical norm of the activity status signal is calculated to determine
whether the signal is consistent with normal physiological activity or
indicative of a sudden and vigorous movement associated with a syncope. A
random occurrence such as a syncope exhibits a high standard deviation,
whereas the more constant signals associated with true physiological
activity and exercise such as walking display a considerably lower
variation from the mean or physiologic statistical norm. Therefore, this
calculation can provide a clear confirmation of what appears to be a fast
VT or VF from the reading of cardiac activity such as an ECG. The mean and
the standard deviation of the activity status signal are determined, and
the latter is divided by the former, by the microprocessor of the
implanted medical interventional device.
For greater reliability, the activity status sensor signal is processed to
compare the standard deviation and the mean over time, which is better
suited to low level activity signals. A running average of the comparison
may be calculated in blocks of time, such as one second each over a
substantially longer time interval such as 32 seconds, for example, and
each block processed on a first in - first out basis, to eliminate minor
perturbations such as noise. This technique is useful not only to
distinguish noise or spasmodic reactions from true exercise, but
additionally to differentiate between different types of physical
exercise. Furthermore, calculation of the standard deviation relative to
the mean of the activity status signals to differentiate distinct types of
exercise can be applied to signals derived from changes in intrinsic
physiological parameters (such as blood temperature, respiration, etc.) as
indicative of exercise, as well.
Therefore, it is another object of the invention to provide techniques for
detecting collapse or evidencing other dramatic hemodynamic changes of the
patient by calculating the standard deviation relative to the mean of
signals derived from an activity status sensor or from any other sensor
representing intrinsic physiological parameters, for confirming ECG
evidence of pathologic tachycardia or fibrillation, to trigger the
application of appropriate electrical waveform therapy to the patient's
heart.
In a method of controlling the therapeutic function of an implantable
antitachycardia device according to the invention, an electrical signal is
generated representing sensed variations of an intrinsic physiologic
parameter indicative of the substantially instantaneous hemodynamic
condition of the implant patient, the mean and the standard deviation of
the generated signal are calculated over a predetermined time interval,
the quotient of the standard deviation and the mean over that time
interval is calculated to determine therefrom a sudden hemodynamic change
in the patient, and the function of the antitachycardia device is
controlled in response to a determined sudden hemodynamic change. Here
again, the intrinsic physiologic parameter to be sensed may be blood
pressure, blood oxygen content, minute ventilation, central venous
temperature, pulse rate or blood flow of the patient, or other known
parameters. The therapeutic function of the device may include
defibrillation, and that function may be triggered upon determination of a
sudden hemodynamic change indicative of a drop in cardiac output exceeding
a predetermined threshold level.
To further enhance the probability that the cardiac activity being detected
warrants delivery of an appropriate therapy, and thereby avoid unnecessary
and undesirable application of shocks, a scoring system may be used in
which various parameters of the ECG and activity status signals are scored
by weighting parameters of the signals according to the significance of
their representations, and then tallied and compared against levels or
degrees of probability that a particular cardiac event of interest is
indicated.
According to the invention, a method of determining the timing for applying
selected antitachycardia and defibrillating electrical therapies to a
patient's heart from an implanted interventional device such as an
automatic implanted defibrillator includes detecting the patient's cardiac
activity to recognize a VT, while simultaneously detecting the current
status of physical activity or inactivity of the patient, determining
whether the recognized VT is physiologic or pathologic by comparing the
detected cardiac activity with the detected status of physical activity or
inactivity, and delivering an antitachycardia electrical therapy when the
comparison indicates a pathologic tachycardia. A further aspect of the
method is weighting the detected cardiac activity and the detected status
of physical activity or inactivity as separate indicia of the degree of
probability that a pathologic tachycardia is occurring, and tailoring the
delivery of therapy according to the degree of probability indicated by
the weighting. Defibrillating electrical therapy is delivered if the
antitachycardia electrical therapy proves ineffective to terminate the
tachycardia coupled with an indication of acceleration into VF.
According to the invention, a method of determining when to apply any of a
plurality of available antitachycardia therapies, including possible
defibrillation, to the patient from the implanted antitachycardia device
includes detecting at least two different physiologic parameters of the
patient, one of which represents electrical cardiac activity of the
patient, processing the detected different physiologic parameters to
obtain a variety of subparameters, scoring each of the processed
subparameters by assigning to it a relative value according to its
importance in signifying the probability of occurrence of a predetermined
hemodynamic event, and applying a selected one of the plurality of
available therapies for treatment of the predetermined hemodynamic event
when the cumulative scoring of the processed subparameters reaches a
preestablished level of probability that a hemodynamic event is occurring
which requires the selected therapy. In the preferred method the other
physiologic parameter being detected represents physical activity of the
patient, and the subparameters obtained from the processing step for that
parameter may include patient rest, patient exercise, patient position,
and/or change of patient position.
Thus, the method of controlling the therapeutic function of the implantable
antitachycardia device may include detecting the posture, position or
physical activity of the patient by use of one or more accelerometers
carried by the patient, "carried" meaning that the accelerometer(s) may be
implanted or, alternatively may simply be worn externally by the patient.
The posture, position or physical activity of the patient detected by the
accelerometer(s) may be used for comparison against the ECG of the patient
to determine whether the patient is experiencing a physiologic tachycardia
or a pathologic tachycardia, and, in the latter event, an appropriate
antitachycardia therapy is applied from the device.
The method of the invention also includes monitoring the cardiac activity
and current status of physical activity or inactivity of the patient
following the delivery of an antitachycardia therapy to determine whether
that delivered therapy protocol succeeded in terminating the pathologic
tachycardia, and delivering another antitachycardia protocol different
from the last delivered therapy protocol when the monitoring indicates
that the last delivered protocol did not terminate the pathologic
tachycardia. The selection of the next antitachycardia therapy protocol
following the determination that the last protocol did not succeed, may be
made from a group of known cardiac pulse and shock therapies or protocols
for breaking VT and VF, programmed into the interventional device, such as
one or more bursts of stimulating pulses, a train of rapid stimulation
pulses, one or more low energy shocks, and a relatively higher energy
defibrillating shock. If the VT progresses, the next therapy or protocol
selected would typically be more drastic than the last, ranging ultimately
up to an appropriately phased high energy defibrillating shock.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, aspects, features and attendant advantages of
the present invention will become apparent from a consideration of the
following detailed description of a presently preferred embodiment and
method, taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a graph or chart illustrating readouts measured in g versus time
from an activity status sensor implanted in a patient;
FIG. 2 is a graph of the change in g relative to time for an event
constituting a sudden vigorous movement by the patient;
FIGS. 3A and 3B are perspective representations of a pair of mercury ball
sensors in fixed orthogonal orientation as implanted in a patient, and an
accelerometer fabricated in an integrated or hybrid electronic circuit,
respectively;
FIG. 4 is a graph contrasting the output of a mercury ball sensor in number
of contact changes per unit of time, for a patient during walking and
during a syncope;
FIG. 5 is simplified depiction of an implantable medical interventional
device with activity status sensor(s) located within the device housing;
FIG. 6 is a graph or chart of sensor contact changes with time relative to
numbered contacts for walking and a syncope, as detected by an activity
status sensor of the mercury ball type;
FIG. 7 is a plot of the quotient of the standard deviation relative to the
mean of the output signal of the activity status sensor, for walking and
syncope;
FIG. 8 is a phantom view of a patient with an implanted interventional
device according to the invention;
FIG. 9 illustrates an exemplary scoring technique to aid in determining the
specific nature of the cardiac event being detected and, accordingly, the
type of therapy or treatment to be delivered where the event is VT or VF;
and
FIG. 10 is similar to FIG. 5, except for the inclusion of an alternative
arrangement in which the sensor is housed in its own separate case and
connected by an electrical lead to a connector on the device housing.
DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD
In the preferred embodiment and method of the invention, the implantable
interventional device utilizes not only a direct sensor of cardiac
activity such as an ECG sensor, but also an activity status sensor adapted
to detect the position and movements of the patient, preferably an
accelerometer or other electromechanical converter, which may be
calibrated for both static and dynamic outputs. The static output will
depend upon the physical position or posture of the implant patient when
inactive (e.g., at rest, or in a state of collapse). Assume that the
activity status sensor is oriented vertically when implanted, and in that
position produces a zero g (i.e., unit of gravity) output. The sensor
produces a +1 g output in one aspect of its horizontal orientation, (i.e.,
one major side down), and produces a -1 g output in the opposite aspect of
horizontal orientation (the other major side down).
In this example, illustrated by the chart of FIG. 1, the activity status
sensor provides a zero reading when the patient is standing during a first
time interval, a +1 when the patient is supine during a second time
interval, and a -1 when the patient is prone during a third interval. This
chart is not intended to show the response of the activity status sensor
to changes in position, such as in the period between the first and second
intervals, but only that different readouts or signals are produced by the
sensor upon detection of different static positions of the patient.
The particular outputs may be modified by calibrating the sensor for slight
deviations of orientation relative to these three positions. If the
patient were lying on her side, the sensor reading would not be +1 or -1,
but a non-zero value. By calibrating the electrical output of the sensor
circuit after implantation, the orientation of the sensor in any position
of the patient will be known from the readout. By employing a second such
activity status sensor with a 90.degree. orientation relative to the first
sensor, each sensor recognizes two of the three mutually orthogonal axes
(X-Y-Z) in three dimensional space, and both together detect a full three
axis orientation, to provide a combined reading uniquely identifying the
patient's position. Thus, the sensor may be used to detect static or
stable (i.e., non-moving) position of the patient, and also to detect
patient activity constituting dynamic movements of the patient, such as a
momentary change of position or continuing movements such as walking,
dancing, bicycling, and so forth.
If the patient's position were to change suddenly from standing up to lying
down, the activity status sensor output would undergo change from a
reading of 0 to +1, which could indicate a syncope. There are other
possibilities, of course, such as tripping and falling, or merely
consciously dropping quickly on a bed. As will be discussed in greater
detail presently, the fact that other possibilities exist, even in a
situation where one sensor, such as the activity status sensor, is used
for confirmation or reinforcement of another sensor--e.g., an ECG
sensor--leads to the use, according to another aspect of the invention, of
a scoring system or other evaluation system by which to assess the
probability that the patient is experiencing a VT, a fast VT, or
fibrillation, by evaluating the signals of both sensors not only
instantaneously but over time.
By analyzing the activity status signal(s), the particular position,
ongoing state of activity, or mere sudden change in position of the
patient can be detected with considerable precision. For example, if the
patient is standing and at rest, the sensor output is 0; but if he
suddenly collapses, the sensor responds with a relatively large spike
output. If the actualvalue of the change, delta g over time (g/t), is
calculated, or the quotient of the standard deviation and the mean of the
continuously monitored activity status signal is computed for that period,
it becomes quite apparent that a dramatic event has occurred, as shown in
FIG. 2.
Concurrent detection of the patient's cardiac activity exhibiting a rapid
heart rate along with such an assessment of the activity status signal
would indicate a pathological tachycardia--either a very rapid VT or even
VF, with apparent loss of consciousness by the patient. The implanted
interventional device would be programmed to respond automatically in
these circumstances by immediately delivering an appropriate treatment.
For example, the device would commence charging its output capacitors to
an appropriate level and, upon reaching that level, deliver the
predetermined (programmed) electrical waveform therapy, constituting a
single or multiple phase shock of appropriate energy content, to the
patient's heart. The distinct advantage achieved in this situation is that
precious seconds are saved, the loss of which might otherwise have caused
a fatality.
It will also be seen that these detection techniques provide improved
discrimination between physiologic tachycardia and pathologic tachycardia,
which can overlap at heart rates ranging from about 130 to about 180 bpm,
than has heretofore been achieved. Indeed, a serious problem that occurs
with ECG detection alone is such overlap between the rates for a fast
physiologic tachycardia and a relatively slow pathologic VT. Some patients
experience slow VT while under medication, and are at risk that, although
of relatively slow rate at the moment, this dangerous tachycardia may
develop (accelerate) into fibrillation. Normally, VF occurs only after VT;
it is rare that the person will experience VF which is not precipitated by
a pathologic tachycardia. The implantable interventional device may be
programmed to respond to a sensed ECG signal indicative of possible slow
VT, coupled with confirmation by the activity status signal indicative of
either collapse or physical inactivity of the patient, by stimulating the
heart with low energy shocks to break the VT and prevent VF from
developing. Alternatively, the programming philosophy may represent a less
conservative approach in which the slow tachycardia and lack of physical
activity of the patient merely define an alert condition of the device in
which the capacitors are charged to the proper energy level, in
anticipation of the possibility that a more dramatic situation may
develop. If it then happens that the delivery of an antitachycardia or
defibrillating shock is warranted, precious time will not have been lost
waiting for the output storage capacitors of the device to be charged.
Advantageously, the use of two complementary sensors in this manner serves
not only to control charging and firing of the implantable interventional
device for treatment of tachycardias and fibrillation, but also to better
evaluate the probability of success of interventional measures. Since VT
normally can be broken by lower energy doses (pulses) discharged from a
device to tile heart than the magnitude of energy needed to terminate VF,
a considerable energy saving is achieved which helps to reduce the size of
the battery and, thus, of the implanted device, or to increase its
lifetime with the same battery capacity. These are important goals in the
development of any battery-powered implanted medical device.
It should be emphasized, of course, that numerous conventional electrical
waveform therapies or therapy protocols may be programmed into the
interventional device for selective application to the heart upon
detection of an applicable cardiac event from the complementary sensors.
For example, these may include single stimulating pulses, stimulating
pulse sequences, stimulating pulse trains of variable repetition
frequency, one or more bursts of stimulating pulses, and single phase or
multiple phase shocks of variable energy content generally greater than
the energy content of the pulses in the other protocols which are utilized
for treatment. In general, the therapy is applied in successively more
stringent protocols until it is successful to break the VT or VF.
Both the degree of difficulty and the likelihood of failure in achieving
defibrillation increase with the length of time that the patient is in
fibrillation. It is crucial to reduce to the maximum practicable extent
the time interval from onset of fibrillation to delivery of the initial
shock, from the standpoints of both the energy required to defibrillate
the heart and the opportunity to successfully defibrillate and resuscitate
the patient. It is considerably easier to interrupt a VT, which may
require delivery of only one joule of electrical energy, than to terminate
VF with the potential requirement of 15 joules or more in each shock.
Correspondingly, resuscitation is much more achievable with a patient who
has been in fibrillation for only a few seconds than with one whose attack
has continued for several minutes. Prompt application of treatment is also
important with a patient who is experiencing either VT or VF and is
fighting against loss of consciousness. If the interval from onset to
delivery of therapy is excessive, e.g., ten to thirty seconds, the patient
may begin to faint, whereas if intervention had taken place earlier the
circulatory system would have been better able to compensate for the fast
heart rate without the patient losing consciousness.
The type of dual sensing used according to the present invention aids the
implanted programmable microprocessor-based interventional device to
better interpret the ECG criteria normally applied to detect pathologic
tachycardias (e.g., heart rate, morphology of the ECG, sudden onset, rate
stability, etc.), because the activity status sensor is complementary,
providing additional information concerning the cardiac event under
scrutiny, rather than merely part of the ECG criteria. The speed with
which a better understanding and interpretation of the ECG is obtained,
through the use of this complementary sensor, vastly improves the
patient's chances of survival by virtue of the lower energy requirement
and higher probability of resuscitation. The activity status sensor not
only increases the probability of detection compared to the use of an ECG
sensor alone, with resulting faster response, but also improves
discrimination between physiologic and pathologic tachycardias especially
in the borderline region from 130 bpm to 180 bpm, thereby better avoiding
needless, painful and debilitating shocking of the heart.
Preferably, the activity status sensor is of the mercury ball type
described in U.S. Pat. No. 4,846,195. Although a single sensor may be
used, it would constitute only an X-Y axes (two dimensional) detector
incapable of uniquely identifying position. For reasons mentioned earlier
herein, it is preferred that two such sensors with fixed orthogonal
orientation for combined detection of all three axes (X-Y-Z) of potential
direction of movement or position of the patient be used. Referring to
FIG. 3A, sensor 10 includes a pair of mercury ball sensors 12 and 13
coupled together at an angle of 90 degrees. This fixed orientation of the
two assures that their combined output signal will properly identify
different specific physical positions of the patient even if the overall
sensor were to undergo a shift in its orientation after implantation in
the patient's body.
It is desirable, however, that the orientation of the sensors after
implantation be such that one of them (sensor 13, in this example) is
approximately horizontal and the other (here sensor 12) approximately
vertical when the patient is standing upright. In the exemplary
configuration represented in FIG. 3A, sensor 12 is somewhat smaller than
and assembled within sensor 13 in the fixed relationship. In practice, the
two position sensors may be separated, but nevertheless fixed in their
orthogonal orientation. Under static (motionless) conditions of either
sensor of the pair, the mercury ball (not shown) is at rest and contacts
specific ones among the set of electrodes (electrical contacts) 14
disposed about the side or floor of the sensor chamber. The electrodes are
connected by respective electrical conductors to the output circuit of the
sensor, each set of electrode locations in conductive contact identifying
a particular position of the patient. As the patient changes position or
engages in ongoing physical activity, one or both mercury balls will roll
about within their respective chambers and make and break contact with the
electrodes. The combined static locations of the mercury balls within the
sensor pair, or their dynamic locations as they make and break connections
between adjacent electrodes (closures and openings with time) is detected
to provide information regarding the physical position, change in
position, and ongoing movements of the patient.
Alternatively, the activity status sensor may be an accelerometer which is
fabricated in silicon or other semiconductor material within an integrated
electronic or hybrid circuit, such as that described in U.S. Pat. No.
5,031,615. A hybrid semiconductor integrated circuit incorporates the
accelerometer as a microminiature mechanoelectrical converter or
transducer of high efficiency and low power consumption. This type of
accelerometer 15, shown in FIG. 3B, has a silicon monocrystalline
substrate 15-1, an epitaxial doped layer 15-2 overlying the surface of
substrate 15-1, and a polycrystalline silicon layer 15-3 sandwiched
between passivating layers 15-4, 15-5 of silicon dioxide. A cavity 15-6 is
formed in the substrate by etching, and portions of the silicon and
passivating layers are removed, forming a rectangular plate 15-7 connected
by four arms to the corners of the cavity. The plate and its arms
constitute the acceleration responsive element. An additional layer may be
provided on the structure with an opening contiguous with the cavity to
allow axial movement of plate 15-7 on its arms, and a protective layer of
glass disposed over the structure. An integrated circuit suitable for
processing the movements of the plate in response to acceleration to
provide the activity status signal may be fabricated in the silicon
substrate, within the region generally below that designated by 15-8, by
conventional semiconductor processing techniques.
In FIG. 4, curve 17 represents the number of contact changes per second
(along the ordinate) in the horizontal mercury ball sensor 13 of FIG. 3A
relative to elapsed time in seconds (along the abscissa) for a subject
wearing the sensor and walking at a rate of 3.2 kilometers per hour
(kin/h) on a 0% grade. Although only one sensor output is indicated,
similar results are obtained for the vertical sensor, albeit they may be
larger or smaller signals depending on extent of movement for its
orientation. The activity status signals from the sensors are processed
with identification of each contact of each sensor, the number of contact
closures and openings for each contact, and the instant of time at which
each closure or opening took place within a specified time interval. It
will be observed that when the patient is walking, the output of the
mercury ball sensor (each sensor) is relatively smooth. In contrast, curve
20 illustrates the output signal produced by the sensor with the subject
feigning collapse (corresponding to a patient experiencing a syncope).
This large spike considerably exceeds the number of contact changes with
time that occurred during a corresponding interval for walking.
FIG. 5 is a simplified diagram of an implantable interventional device such
as defibrillator 25 which is of conventional construction, except as will
be noted presently, to provide both antitachycardia and defibrillation
therapies. The defibrillator components including batteries 27,
electronics 29 (e.g., including a microprocessor, memories, etc.) and
output capacitors 30 for storing electrical charge in variable quantities
according to the amount of electrical energy to be delivered for shocking
the heart to provide the desired therapy. According to the invention, an
activity status sensor is incorporated within the defibrillator in the
form, for example, of orthogonally oriented mercury ball sensors 35, 36
substantially as described in FIG. 3A except that the two may be
separately affixed to maintain that orientation. The two are housed within
but mechanically isolated from the case 32 which houses all of the other
components of the defibrillator, to avoid sensitivity merely to pressure
on the case. Alternatively, the sensor pair may be housed in its own
biocompatible hermetically sealed case 38 for implantation in the patient
in a location separate from the defibrillator case, as shown in FIG. 10,
in which like components are designated by like reference numbers.
The lead 40 for connecting the separately housed sensor implant 38 to the
electronic control circuitry of the implanted defibrillator 25 may be a
multiple contact type such as that disclosed in U.S. Pat. No. 4,971,057,
to facilitate the signal processing. The defibrillator case 32 includes a
header 43 with electrical connectors for the lead associated with the
activity status sensor implant (if in a separate implantable case) and for
the lead(s) connected to the defibrillating and other electrodes for
delivering therapy and sensing cardiac activity. An ECG sense amplifier
and related processing circuitry included within electronics 29 of the
defibrillator provide an ECG signal for detecting rapid heart rates and
other cardiac activity.
In operation, the implantable medical interventional device (defibrillator
25, in this example) is adapted to intervene upon detection of cardiac
activity of the implant patient indicative of VT or VF by successively
applying to the heart selected ones of several different preprogrammed
electrical waveforms conventionally utilized as protocols for treatment to
break VT or VF, respectively, until the monitored cardiac activity
indicates that the treatment has been successful. The activity status
sensor detects physical activity and inactivity of the patient to
complement detection of the patient's cardiac activity for confirming that
a detected VT is a pathologic tachycardia rather than a physiologic
tachycardia. The microprocessor in the device responds to such
confirmation to select an appropriate one of the preprogrammed protocols
stored in memory for treatment, with application of the selected
electrical waveform to the heart via the output circuit of the device and
the leads. In the example of the defibrillator, the shocks intended to
defibrillate the heart are produced by charging the output capacitors of
the device to the predetermined energy level, and then discharging them
through the heart in the desired phased waveform, in a conventional
manner. Certain other aspects of the device operation will be addressed in
the description of FIG. 8 below.
FIG. 6 is a chart of the number of contact changes with time (ten second
intervals) plotted on the ordinate against the designated numbered
contacts of each sensor on the abscissa for walking (curve 52), slight
position changes (curve 50) and feigned syncope (curve 54) of a healthy
volunteer subject carrying a horizontally-disposed mercury ball sensor.
Despite the relatively small number of contact changes with time for the
syncope compared to an activity such as walking, when plotted against
time, as in FIG. 4, the maximum value of the syncope significantly exceeds
that of the walking activity.
FIG. 7 is a bar chart or plot of the quotient of the standard deviation
divided by the mean of the signal produced by the horizontal mercury ball
sensor for walking and collapse, and illustrates the clear distinction of
a syncope from other types of physical activity by this technique. For the
walking activity (bars 60) the discrete calculations produce consistently
low values. In contrast, the syncope (bars 62) has considerably larger
discrete values and average value.
By calculating the quotient of the standard deviation (the variance) and
the mean of the changes of contact "makes" and "breaks" by the moving
mercury ball of the sensor, continuing activity can be discriminated from
sudden changes of position, regardless of the extent of the changes over
time. The standard deviation relative to the mean is a low value for
ongoing activity, and a high value for brief random movements or noise.
Hence, this calculation can be used to clearly differentiate between
signals which are relatively continuous and signals which are attributable
to sudden brief vigorous movement, for example a collapse with loss of
consciousness. All calculations are readily performed by the
microprocessor of the device.
Heretofore, the inability to fully assess the hemodynamic consequences of a
tachycardia in different patients has made it difficult to provide a
device universally adaptable to determine whether and when a particular
antitachycardia therapy should be delivered. One patient may be able to
tolerate an elevated heart rate of 180 bpm with a rapid decline of
systolic blood pressure to 65, for example, whereas another patient,
because of stenosis and weak cerebral profusion, may suffer loss of
consciousness and respiratory functions with a tachycardia rate of 160 bpm
and systolic blood pressure of 70. Measurements of heart rate, stroke
volume, cardiac output, and even blood pressure do not fully delineate
cerebral status for each individual patient. Hemodynamic parameters may
appear to be within a normal range or not life-threatening, but they do
not provide a true indication of the cerebral function of the patient.
With the present invention, by comparing the ECG signal with the activity
status signal, however, the occurrence of a slow pathologic tachycardia is
recognized by the fact that the elevated heart rate is present with little
or no physical activity by the patient, as distinguished from a
physiologic tachycardia where coincidence of rapid heart rate and
pronounced physical activity is evident from the outputs of the two types
of sensors. Also, loss of consciousness by the patient is readily detected
from the above-described calculation performed by the microprocessor, of
variances relative to mean of the activity status signal to indicate the
need for a more drastic therapy in response to a dramatic cardiac event
(e.g., syncope). Thus, in the system of the present invention, in addition
to the improved discrimination of pathologic versus physiologic
tachycardias, the detection system provides an indication of hemodynamic
consequences which warrant application of a specific antitachycardia
therapy.
FIG. 8 illustrates the defibrillator 25 implanted in a patient 70, with
lead/electrode assemblies 72, 73 for an epicardial patch electrode 75 and
an endocardial counter-electrode (not shown). The patch electrode overlies
an appropriate region of the epicardium and the counter-electrode in
positioned in the right ventricle, for efficient delivery of the high
voltage, high energy shock waveform to the heart 77. Implantable
defibrillation apparatus of various types is known in the art, and the
specific type is not critical to the implementation of the present
invention except as otherwise expressly stated herein.
The control means of the conventional microprocessor of the device includes
computing means responsive to detected physical activity of the patient by
the activity status sensor for recognizing from the detected activity the
occurrence of a sudden vigorous movement associated with a syncope by
calculating the variances relative to the mean of the detected physical
activity. The microprocessor control means further includes activation
means which responds to such recognition for selecting a defibrillating
electrical waveform therapy or protocol for immediate application to the
patient's heart.
The system and method of the invention further provide a tallying technique
by which subparameters of the detected cardiac activity and the detected
physical activity or inactivity (or other sensed physiologic parameters,
as stated earlier herein) are weighted according to their respective
significance in identifying the cardiac events of interest, to determine
from the total weighting in a given interval of time (including, in the
lower limit, an interval which is substantially instantaneous) the
likelihood that physiologic tachycardia, pathologic tachycardia, or
fibrillation is being detected in that interval.
FIG. 9 is a simplified diagram illustrating such a scoring system for
enhancing the probability of detecting a cardiac event requiring
antitachycardia therapy, including, among other protocols, defibrillating
shocks. A number of subparameters of the sensed cardiac activity and
physical activity or inactivity (from sensors 80 and 82) may be designated
for use in this system. For example, subparameters 1 through 5 may be (1)
absolute heart rate or absence of QRS complexes, (2) relative heart rate
change in absolute numbers (bpm), (3) relative heart rate change over time
(bpm/time), (4) rate stability, and (5) ECG morphology criteria. These
subparameters may be assigned possible scores ranging from one to ten
each, depending on their relative importance, sensitivity and specificity,
and the extent to which they correspond to criteria indicative of a
cardiac event requiring an available therapy. Other subparameters 6, 7 and
8 may be scored, for example, based on (6) patient rest or activity
(exercise), (7) patient position, and (8) change of position.
The individual subparameter scores are tallied and, for example, a total
score of up to 20 is programmed to indicate a high probability of a slow
VT, a total score of 20 to 30 to indicate a fast VT, and a total score
exceeding 30 to indicate VF. The programming of the implanted device may
now be set up to respond to scores within the established ranges by
delivering the appropriate therapy including operation selecting whether
the device charges its output capacitors to deliver a pulse or pulses
having up to the maximum energy level or some predetermined lower level.
In the specific example of FIG. 9, subparameters 1-5 which are allocated
to the ECG signal have respective scores of 0, 0, 0, 1, 2, and
subparameters 6-8 which are allocated to the activity status signal have
respective scores of 1, 0, 1. Here, the total score is 5, which indicates
a sufficiently slow heart rate as to not require the delivery of
antitachycardia therapy by the device at this time. Continued monitoring
by the complementary sensors may lead to scoring changes that will
determine whether therapy should be applied.
Although certain preferred embodiments and methods have been disclosed
herein, it will be apparent to those skilled in the art from a
consideration of the foregoing description that variations and
modifications of the described embodiments and methods may be made without
departing from the true spirit and scope of the invention. Accordingly, it
is intended that the invention shall be limited only to the extent
required by the appended claims and the rules and principles of applicable
law.
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